U.S. District Judge Florence-Marie Cooper issued a temporary injunction Monday, rejecting a Navy request that she dismiss a lawsuit filed by the Natural Resources Defense Council.
The council says the Navy planned to use so-called mid-frequency sonar over thousands of square miles of ocean in an area rich in marine life. The council contends the high-power sonar causes marine mammals to beach themselves and leads to other harm.
The injunction will force the Navy to comply with federal environmental laws protecting marine life, said Joel Reynolds, a senior attorney for the NRDC.
"We don't take issue with the Navy's judgment that it needs to use this technology," Reynolds said. "Our concern is when they test and train, they do so in a responsible manner."
The Navy said it would appeal the injunction, arguing that sonar is a vital tool in detecting submarines.
"To the extent this court decision prevents us from using active sonar, it potentially puts American lives and our national security at risk," the Navy's Third Fleet commander, Vice Adm. Samuel Locklear, said in a statement.
The Navy maintains that it already minimizes risks to marine life. It has monitored the ocean off Southern California for the 40 years it has employed sonar without seeing any whale injuries, the Navy said in a news release.
The Navy has planned a series of 14 training exercises using sonar. It says it has already carried out three of these and has found no evidence of strandings, injuries or behavioral disturbance to marine mammals.
Reynolds said the ban would remain in effect until his organization's lawsuit is settled.